Stratigraphic and regional variation of the petrographic and chemical properties of the Tradewater Formation coals and surrounding rocks in western Kentucky

The stratigraphic and regional variation of petrographic and chemical properties within the coals of the Upper Carboniferous Tradewater Formation and surrounding rocks in the Western Kentucky coal field were analyzed with the intent of constructing a depositional model for the occurrence of these low sulfur coals. Cores were megascopically described, and coal samples were analyzed for maceral, ash, and sulfur contents. These data were then analyzed to determine regional variation within the study area, as well as vertical variation within single coal columns. Sedimentological data from core logs indicate that the majority of the Tradewater rocks consist of irregularly distributed, coarsening-upward, fine-grained detrital material that was deposited in shallow bodies of water. Fossiliferous shales and limestones indicate a marine influence. Less common coarse-grained, fining-upward sequences appear to represent deposits of meandering or braided channels. Like the detrital rocks, the coal seams are irregularly distributed and exhibit substantial variation in petrographic and chemical properties which reflect changes in the Eh and pH of the coal swamp waters. These individual swamps were relatively limited in extent and probably occupied a low-lying coastal area. The relatively high vitrinite content of most of the coals suggests a reasonable degree of preservation of decaying plant materials. The study of benched samples from surface mines suggests a distinct dichotomy between swamps that were in more or less continuous contact with sulfate-rich marine or brackish water and those in which peat accumulated in a dominantly fresh-water setting. Most of the latter show a pattern of upward increasing sulfur content and decreasing vitrinite content, indicating increasing influences of oxygenated water that would encourage microbial action and which would degrade the peat and increase the tendency for sulfide precipitation. The high sulfur coals do not display this variability. The high rates of lateral variability encountered in the data suggest that future study should concentrate on smaller areas where variation can be completely documented.